The invention relates to an expansion valve, in particular for an air-conditioning system operated with cooling medium, with a valve seat and a valve-closing element, which closes a passage opening between a cooling-medium inlet opening and a cooling-medium outlet opening, and with an actuating device which acts on the valve-closing element and opens and closes the passage opening.
In vehicles, it is becoming increasingly common to equip air-conditioning systems with at least one additional evaporator in order for it to be possible to cool separately at the front and at the rear or on the left and the right side, for example. To avoid unnecessary energy consumption, it is desirable for it to be possible to switch the additional evaporator off when there is no requirement. However, separate switch-off valves arranged in the cooling-medium line are relatively costly and require additional construction space.
Expansion valves which combine such separate switch-off valves in one expansion valve have therefore already become known. To this end, to switch off the additional evaporator circuit, a flowthrough opening arranged in the expansion valve is closed completely by a valve-closing element arranged in a valve seat. The activation of the valve-closing element is effected by means of an actuating device which comprises a thermohead and a transmission pin which acts on the valve-closing element. In this way, a mass flow flowing through the passage opening is controlled.
The previously known expansion valves are intended for use for high capacities. The flow rate of the cooling-medium mass flow is limited by a maximum working lift of the valve-closing element, an annular gap cross section formed between the transmission pin and the passage opening always being designed to be larger than a cross-sectional area between the valve-closing element and the valve seat in the working lift. By virtue of this, valve regulation takes place via the ball-valve seat cross section. Such expansion valves have a large working lift and are used for mass flows >100 kg/h at a condensation temperature of 38° C., undercooling of 5 K and an evaporation temperature of 0° C., for example. Such expansion valves are very inaccurate for the control of small mass flows. Moreover, the efficiency of a cooling-medium system decreases if a small mass flow is operated with such expansion valves.